Textural elements analysis in the Jajarm karst bauxites: Composition and conditions of formation

The Jajarm karst bauxite deposit is located about 175 km southwest of Bojnourd. The deposit has been developed as a stratiform horizon along the contact of Triassic dolomites and the Jurassic shales and sandstone. In this study, the textural elements of the Jajarm bauxites are classified into matrix and separated textures based on morphological and genetic criteria. Pelitomorphic and microgranular textures are the most abundant matrix textural elements and ooids, pisoids, pelletsand secondary occurrences are the main separated textural elements. Textural analyses indicated both allochthonous and autochtonous origins for the Jajarm bauxites. Detailed petrographic studies allowed the recognition of two types of ooids and pisoids based on morphological features and laminations. Type A is characterized by thin, regular and continuous lamination, whereas type B has thick, irregular and discontinuous lamination. Ooids and pisoids are mainly composed of alternating kaolinite-hemetite and diaspore-geothite laminae in which Al2O3 concentration increases towards the outer laminae. Fine-scale alternation of these laminae in the ooids and pisoids implies the climatic fluctuations of wet and dry seasons. Secondary textural elements are mainly observed as veins enriched in SiO2 and Al2O3.